Three Types

There are essentially three types of ballistics; internal, external, and terminal (Heard, 2008). Each has its own important role for us to understand and tie back to our investigations throughout a forensically sound practice.

Internal Ballistics

Internal ballistics is as it sounds, it is the ballistic changes from the moment that primer strikes and the round leaves the barrel (Heard, 2008), which surprisingly for as short as that is, is actually a huge portion of what we are concerned with as investigators as there is where the round get tied to the firearm and we are able to say round A came from gun A. Mostly during this phase we are studying, propellant pressures, acceleration of the projectile, muzzle velocity, and recoil (Heard, 2008). If you truly want to get into the weeds you could look at primer ignition time, primer pressure/time curves and temperature but those are not covered in our text book and are a bit more advanced that our current objective (Heard, 2008).

External Ballistics

External ballistics covers the flight of the projectile, all the way from the muzzle of the firearm to the target (Heard, 2008). Overall this is a terribly complicated subject, rounds do not just come out and fly straight, as they leave the barrel they are actually on an upward trajectory then peak out and start to fall towards the ground. This is why if you zero your rifle, let’s say a standard AR-15 using 55 grain round with a 1/9 left hand twist, at 50 yards then move up to about 10 yards, you will notice you need to aim 2 inches high from where you want your impact to be, part of this is bore over axis, part of this is the trajectory of the round.

Terminal Ballistics

Terminal ballistics, as it sounds involves the termination of the ballistics, what it does upon impact with its target (Heard, 2008). Does the round mushroom upon impact and have minimal penetration such as a standard hollow point round? Or does the round maintain its shape and penetrate harder armor and goes through and through its target such as AP rounds. You will not get this from your paper targets, go on YouTube with the name of your favorite round and type in gel test; you will see what I am talking about.

Stellate Tearing

Stellate tearing is a way to determine the distance from the weapon to the skin. When trying to determine length of distance from a subject to a victim it is important to understand what you wound pattern should look like. There is both the entrance wound which you will have in all shootings, and then the exit wound depending on the type of weapon/ammo used if trying to determine range there will more than likely be an exit wound especially for hard contact and loose contact shots.

Managing the pressure

For a hard contact shot at the entrance wound you can expect to see clothing singed, muzzle impression, as a hard contact shot means the muzzle is pressed against the intended target (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008). Further the impact site you can expect to see is what you more often expect with an exit wound, the entrance cavity has nowhere for the pressure and explosion of the firearm to go and rips at the skin creating a much larger hole than the round (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008).

Often times, the discharge residue follows the round into the wound channel (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008). Due to this closed entrance, the pressure and gasses from say a sternum shot try to exit the body and often times leave through the entrance wound creating the stellate tear with blood and bone often exiting out the entrance wound (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008).

Expectatations

For close or loose contact wounds you can expect a burning, soot and powder, abrasion ring, usually obscured, angle of entry (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008). You can expect to see the same as above except for the part of the muzzle that is not in contact with the skin, pressure will release from there creating the rip in the tissue and the exit hole for some pressures. Because of this you get a much higher chance of seeing powder residue and such (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008). Loose contact is when the muzzle is pressed to the body but not fully sealed say nudging them with the weapon and shooting them rather than pressing the weapon into them with direct attempt.

 Not so much stellate tearing but the other ranges are:

Intermediate range, you can expect to see abrasion ring which might be obscured, GSR on the victims clothing, angle of entry (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008). Further you can expect to see some stippling/tattooing, with flake powder burns, typically 18-22 inches away, large irregular, red/brown spots (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008). For flattened ball powder 36-40 inches away, less dense red spots, and for ball powder up to 48 inches dense red pinpoint pattern, this is the powder burns specifically on the target (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008).

Summary

With distant wounds you should see a visible abrasion ring, bullet wipe as it passed through clothing or skin this is usually a sign of homicide as a suicide cannot be accomplished from this distance without assistance (Heard, Range of Firing Estimations and Bullet Hole Examinations, 2008).

References:

Heard, B. J. (2008). Handbook of Firearms and Ballistics : Examining and Interpreting Forensic Evidence. Austin: John Wiley & Sons, Inc.

Heard, B. J. (2008). Range of Firing Estimations and Bullet Hole Examinations. Oxford: John Wiley & Sons, Ltd.

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